Image converter tubes with improved dust screen and diaphragm means



P" 968 l F. GUYOT ETAL 7 IMAGE CONVERTER TUBES WITH IMPROVED DUST SCREENAND DIAPHRAGM MEANS Filed Jan. 10, 1967 2 Sheets-Sheet 1 L UCIEN E60707: BE)? THY/VP M. PEI/1P0, & FRANCIJ S/PO Apl'll 16, 1968 GUYOT AL3,378,714

IMAGE CONVERTER TUBES WITH IMPROVED DUST SCREEN AND DIAPHRAGM MEANSFiled Jan. 10, 1967 2 Sheets-Sheet :1

United States Patent 0 f 3,378,714 IMAGE CGNVERTER TUBES WITH IME RUVPEDDUST SCREEN AND DIAPHRAGM MEANS Lucien F. Guyotand Bertrand M. Driard,Paris, and Francis Siren, Epernon, France, assigncrs to CompagnieFrancaise Thomson Houston-Hatchiriss Brandt, Paris, France, acorporation of France Filed Jan. iii, 1967, Ser. No. 688,314 (:iaimspriority, application France, Han. 18, 1966, 46,245; Mar. 11. 1.966,53,055 11 Claims. (Cl. Sid-65) AiifiTRAQT OF THE DILOSURE An imageconverter such as a brightness amplifier, having a fine-mesh conductivewire screen extending across the electron path beyond the lens and aheadof the viewing screen, for protecting the inner face of the viewingscreen against dust particles generated in the major cavity of the tube.The dustshield screen is positioned to cause minimal distortion of theequipotential surfaces of the lens, and is mounted on a supportingdiaphragm plate which affords protection against stray electrons.

Sumirmry of the invention This invention relates to image convertertubes, particulariy for use as bri htness amplifiers, as widely used inX-ray work and for other purposes. Such a tube generally comprises aphotocathode at the input end of an evacuated glass vessel constitutingthe tube envelope, and an anode provided with a fluorescent screen atthe output end. An electron-optical system including an electrostaticlens is interposed in the path of the photo-electrons from the cathodeto the output screen. When suitable radiations, e.g., X-rays, aredirected at the pho-tocatlrode to form a so-callcd primary imagethereon, electrons are emitted by the photocathode in a patterncorresponding to said primary image. These electrons are accelerated andfocussed by the eiectron-optical system towards the anode and form asecondary image on the output screen, the secondary image having agreatly increased brightness as compared to the primary image.

Inthe operation of this type of tube, considerable difficulties havebeen experienced owing to the unavoidable inclusion of small particlesof foreign matter in the sealed evacuated tube envelope, for reasonsthat will be later described in greater detail. When such floating dustparticles settle on certain active surfaces of the assembly, the qualityof the image can be gravely impaired.

The invention comprises a dustshield member in the form of an extremelyfine mesh screen of conductive wire, positioned beyond the electrostaticlens and ahead of the output image screen of the image converter tube,so as to prevent the passage of a major amount of foreign particles thatmay be present in the main-part of the tube cavity, towards the outputscreen, and thereby protect the surface of said output screen from suchparticles that would otherwise settle on it; the dustshield screen is,at the same time freely traversable by the photo-electrons. According toPatented Apr. 16, 1968 ice the invention, the dustshield screen is sopositioned, at a predetermined axial distance from the plane of theelectrostatic lens section of the electron-optical system; of the tube,that when the conductive wire screen is electrically connected to theanode of the tube, it will not appreciably distort the equipotentialsurfaces of the lens and will therefore not appreciably alter the opticsof the tube. It has been determined that for this purpose, an optimalposition for the dustshield screen is such that its plane issubstantially tangential to the equipotential surface corresponding toabout 0.95 times the potential. difference present across the lens, asmeasured in the absence of the dustshield screen of the invention.

According to a feature of the invention, the dustshield screen ismounted across the aperture of a diaphragm plate mounted at the positionjust indicated, said diaphragm plate serving both as a strong mechanicalsupport for the delicate wire screen, and as a diaphragming meanspreventing the great majority of stray electrons from entering theuseful beam of photoelectrons and forming undesirable spots on theimage.

Description The problem of floating particles of matter in the sealedenvelope of an image converter tube is one that has seriously limitedthe effectiveness of high-power brightness amplifier tubes in recentyears, and has grown more and more acute as the performancecharacteristics of the tubes have been heightened. The greater thefactor of brightness amplification achieved, the worse will be the localimpairment of the image caused by a minute foreign particle. In acopending US. patent application Ser. No. 335,857, filed Ian. 6, 1964,and which is now US. Patent No. 3,304,455, issued Feb. 14-, 1967 andassigned to the same assignees, means have been disclosed for sealingthe input and output sections of the tube against the ingress of foreignparticles. The means of that earlier application overcame an importantcause of image-disturbance by foreign particles, in two critical regionsof the image tube, but provided no safeguard whatever against particlessettling on the input side of the output image screen. While this was ofonly minor consequence in many good tube constructions, at the time theearlier application was filed, the recent development of high-resolutionbrightness amplifier tubes using very fine pencils of photoelectrons hasseriously increased the relative effect of particles settling on theinput side of the output image screen.

It is an object of this invention to provide an image tube, such as abrightness amplifier, which will be more fully and efliciently protectedagainst the scourge of floating particles than was heretofore possible.Another object is to provide a dust screen for screening the input sideof the target screen of an image tube from a major number ofcontaminating particles, without at the same time affecting to anappreciable degree the normal operation of the tube; a specific objectin this connection is so to position the dust screen that it will notsubstantially distort the equipotential surfaces of the electrostaticlens of the tube, whereby the over-all electron optical system of thetube can remain. unchanged. It will, for example, be possible to attachthe novel dust-screen member of this invention to an existent brightnessamplifier tube without any other appreciable modification to the tubeconstruction.

A further object is to associate the novel dust shield or screen memberwith an apertnred diaphragm plate which, at the same time as it servesas a strong and convenient mount for the fine mesh dust screen, alsoacts to protect the output image against spurious electrons in aparticularly critical region of the electron beam.

The invention will now be disclosed in detail in respect to exemplaryembodiments thereof illustrated in the accompanying drawings, wherein;

FIG. 1 is a schematic view, in axial cross section, of a brightnessamplifier tube illustrating the principle of the invention;

FIG. 2 is a large-scale diagrammatic view of the lens section of thetube illustrating the positioning of dust screen of the invention withrespect to the equipotential surfaces of the lens;

FIG. 3 is similar to FIG. 1 but shows the dust shield member combinedwith a diaphragm according to a preferred embodiment of the invention;and

FIG. 4 is a large-scale view, in cross section, of one practicalconstruction of the dust screen and diaphragm assembly of the invention.

A brightness amplifier tube as schematically shown in FIG. 1 comprises asealed evacuated glass envelope 5 having a photocathode assemblygenerally designated 4 at its input end, herein the upper end of thetube, and a fluorescent screen 3 at its lower output end. Anelectronoptical lens system is interposed between the photocathode 4 andthe output screen 3 for accelerating and focussing photo-electronsemitted by the photocathode 4 onto the screen 3. This electron-opticalsystem includes, as shown the pair of coaxial annular electrodes 1 and2., and preferably an axial accelerating electrode positioned ahead ofthe electrode 1, which accelerating electrode is of cylindrical shape asshown at 40. The entire system will generally present substantialsymmetry of revolution about the vertical center axis of the tube.Operating potentials are applied from a suitable high-voltage source tothe various elements of the system by way of connections passed throughsealed nipples of the tube envelope 5. The connections include aconnection wir 31 soldered to photocathode land passed through sealednipple 10, wire 32 connected to first electrode 1 and passed throughnipple 11, and wire 33 connected to the second electrode or anode 2 andpassed through nipple 12. Electrode 40 has a connecting wire not shown.Some of the above enumerated elements will now be described in somewhatgreater detail.

The cathode assembly 4 comprises a downwardly concave, dished element oftransparent material, e.g. glass, having a photoemissive coating formedon its concave output side. Such a coating may comprise alkali metalssuch as potassium, caesium, as well as antimony and/ or other elements.One conventional and convenient method for applying this photoemissivelayer to cathode member 4 is to use a process of vacuum-evaporation andcondensation in situ during evacuation of the tube envelop 5. For thispurpose, there is provided one or more so-called evaporator receptaclesone of which is schematically shown at 14, suitably mounted in a side ofthe envelope. The receptacles 14 are small tubes made of sheet metal andcontaining the metallic and/or other substances of low boiling pointwhich are to be evaporated and recondensed on cathode 4 to provide thephotoemissive coating thereon. An electric heater resistance 35 is shownclosely associated with receptacle 14 and is connected with a suitablesource of voltage, not shown, through a wire 34 led out of the tubeenvelope by way of a nipple 34. Receptacle 14 initially contains acharge of suitable alkali metal and/ or other reagents, and during themanufacture of the tube heating current is applied over wire 34 toresistor 35 to evaporate the charge which then breaks out in vaporizedform through gaps in the walls of receptacle 14 and the vaporrecondenses on the surface of photocathode 4. A movable shutter deviceschematically shown .4 at 15 serves to prevent the vapours from enteringthe lower section of the tube envelope inside the electronoptics lensassembly and settling over the surfaces of the lenses and specially thesecondary screen 3 where the resulting metal coating if allowed to formwould impair the operation of the system. For this purpose the movableshutter device 15 may consist of a pair of generally semicircularshutter elements movable between the sealing position schematicallyshown, in which the elements overlap and provide a sealing partitionacross the top of annular electrode 1, and an open position in which theshutter elements are separated. A simple spring-latch mechanism, notshown, initially holds the shutter elements in the sealing positionillustrated until the evaporation-condensation process described abovehas been completed. After that the shutter device is displaced to itsfinal open position, as by placing the tube bodily upon a rotating jigand rapidly spinning the tube to move the shutter elements outwards bycentrifugal force, the shutter elements then being permanently retainedin their open position by suitable spring means or otherwise. A detaileddescription in the assignees German utility model specification No.1,888,686, refers to this shutter means.

Annular electrode 1 has a larger-diameter upper section, across whichthe movable shutter device 15 just described is mounted, and asmaller-diameter lower section terminating in an inturned flange asshown. The second electrode or anode 2 is generally cylindrical and hasan inturned flange at its upper end which is spaced from the inturnedflange at the lower end of electrode ll. Anode 2 has an o-utturnedflange at its lower end near the bottom end wall of the tube envelope 5,and has a cross wall 6 soldered across said outturned flange, the crosswall 6 being apertured at its center to support the secondary screen 3.In the construction here shown, the first and second electrodeassemblies 1 and 2 are assembled into a sub-unit by means of a set ofspacer posts such as 8 of insulating material, which may be three innumber, and have their respective ends joined to a horizontal outer wallportion of electrode it, and to the outturned flange at the lower end ofelectrode 2, as shown. This sub-unit is held in position within the tubeenvelope 5 by means including a metallic ring 9 having its flanged lowerend joined to a suitable shoulder section of the tube envelope 5, asshown, and having its flanged upper end soldered tothe horizontal wallsection of electrode 1 around the spacer posts 8.

The secondary screen 3 may comprise a flat plate of transparentmaterial, e.g. glass, having its upper or input face coated with asuitable fluorescent layer such as a phosphor composition of zincsulfide and activator additions, as well-known in the art.

In the operation of the tube, the photocathode 4, electrode 1 and anodeZ are connected by way of the conductors 31, 32 and 33 to respectivepotentials V V V such that the first electrode potential V is moderatelypositive relative to photocathode potential V and the anode potential Vis strongly positive relative to first electrode potential V In theseconditions the spaced adjacent end sections of the electrodes 1 and 2define a positive electrostatic lens in respect to electrons issuingfrom the photocathode 4. When a primary image is formed by means ofX-rays or other suitable radiations on the upper surface of photocathode4, the photoemissive layer on the under surface of the photocathodeemits photo-electrons whose rate of emission at each point of saidsurface area corresponds with the intensity of the incident radiations,i.e. the brightness of the primary image, at that point. Thephoto-electrons are accelerated and focalized by the electron-opticalsystem, including the lens just referred to as defined between theadjacent ends of electrodes l and 2, and thus converge on to secondaryscreen 3. The electrons excite the fluorescent surface coating of screen3 and thereby form on said screen a secondary image accuratelycorresponding in pattern to the original primary image. This secondaryimage is of somewhat reduced size as compared to the primary image, butis of greatly increased brightness. Thus, when the secondary image ismagnified optically or otherwise to restore it to the initial size or toa size substantially increased over that of the primary image, a netgain in brightness of several thousand times can still be obtained.

The over-all shape of the electron beam within the tube fromphotocathode 4 to secondary screen 5 can be visualized in a generalmanner as constituting a cone 16 having an apex or cross-over point 0substantially in the plane of electrostatic lens defined betweenelectrodes 1 and 2. Hence, the secondary image on screen 3 is invertedrelative to the primary image on photocathode 4.

In the use of brightness amplifier tubes of the general type describedherein, considerable trouble has been caused by the presence of floatingparticles of matter scaled within the tube envelope. Such foreignparticles may be due to many contaminating causes, including dust motesdrifting in the atmosphere of the construction shop and which may becomeelectrostatically charged so that they cling to the interior surfaces ofthe tube and cannot be removed during evacuation, but become sealedwithin the tube. Particles of matter are also liable to be detached fromthe surfaces of the various elements sealed inside the tube duringprocessing of the tube and manipulation both in manufacture and insubsequent utilization.

In copending application Ser. No. 335,857 filed J an. 6, 1964 andassigned to the same assignce as the present application, dust-shieldmeans have been described which prevent such floating particles fromsettling on the outer or output side of the secondary screen 3, as wellas on the outer or input side of the photocathode i. As disclosed in theco-pending application, the dust-shield for preventing the floatingparticles from settling on photocathode 4 comprise an annular sealherein designated 39, and the dustshield preventing the particles fromsettling on the outer side of secondary screen 3 include a frustoconicalannular sealing member herein generally designated 38, having one endsealed to the under surface of electrode cross wall 6 and its lower endsealed to the inner surface of the end wall of tube envelope 5. Suchdust shield means have substantially reduced the adverse effects offoreign particles present within the tube on the final image providedthereby, but has not eliminated such effects since it does nothing toprevent particles from settling over the inner (here upper) surface ofthe secondary image screen 3. Particles present at this location do notaffect the quality of the image too seriously in the case of image tubesin which the electron beam lid is relatively wide, because the widerange of electron incidence angles then ensures that there willgenerally be enough electrons to strike the phosphor layer of the screenbeneath any settled particle. However, in recent high-performance tubesin which the electrode geometry and distribution of voltages are soselected as to create a fine pencil of electrons 16 in order to increasethe resolution of the tube, this no longer holds true, and particlessettling on the inner surface of output screen 3 assume a preponderantimportance. Because of the size reduction of the secondary image even aminute fragment on the screen surface can appear as a spot ofappreciable size marring the secondary image.

Because of the obvious necessity of keeping the inner space of the tubeclear for the passage of the photo-electrons through it, it has notheretofore been possible to protect the inner surface of the secondaryscreen from floating particles, otherwise than through the exertion ofutmost care to avoid the introduction of such particles in themanufacture of the tube. Since however the total absence of foreignparticles introduced from the outside and/or detached from the interiorof the tube is impossible to achieve in practice, their ill-effects havehad to be accepted as a necessary evil.

It has been found in accordance with this invention that the innersurface of the secondary screen can be effective- 1y shielded against amajor amount of foreign particles without substantially interfering withthe flow of the photo-electrons through the tube, if an inner dustshield is provided in the form of a fine-mesh wire screen or grid ofsuitable construction, positioned in a manner that will now bedescribed. As schematically shown in FIG. 1, the dust shield, generallydesignated 7, extends across the cylindrical wall of the secondelectrode or anode 2, a substantial distance below the upper end of theanode.

In order for such a screen to be effective as a dust shield, its meshsize should be small enough to arrest a majority of the particlescapable of adversely interfering with the secondary image, and thisobject can be considered as attained when the mesh opening does notexceed about 80 microns.

If the wire screen '7 has a mesh opening substantially larger than thisvalue, it is found that it is liable to be traversed by particles which,on settling on the surface of output screen 3, will produce detectabledark spots in the output image; that is, the wire screen 7 will not thenfulfill its dust-shielding function. It is to be noted that a wire meshof the type here contemplated, and having a mesh opening of 80 micronsas just indicated, has a mesh number, or fineness, of about 250 linesper inch. This linesper-inch value therefore constitutes a lower limitfor the fineness of the wire mesh suitable for use in the invention.

On the other hand, an upper limit to the fineness of the wire screensusable herein is set by the condition that the wire screen 7 should notlower the efficiency of the tube by arresting too many of the electrons.In other words, the ratio of the clear surface area to the total surfacearea of the screen should not be too low. This ratio is defined as theoptical transparency ratio, and it decreases sharply as the fineness ofthe mesh, in lines per-inch, increases or, in other words, as the meshopening decreases. The invention uses wire screens having an opticaltransparency ratio not less than 5 0%, preferably not less than 60%. Itis noted that a wire screen having an optical transparency ratio of oractually has an electron transparency ratio which is much higher, beingof the order of to or more, so that the efficiency of the tube is notseriously impaired. In view of the optical transparency condition, asuitable upper limit for the fineness of the wire mesh usable herein isabout 750 lines per inch, and correspondingly a suitable lower limit forthe mesh opening is about 25 microns.

A preferred range of mesh fineness values used according to theinvention is from 400 lines per inch to 500 lines per inch, thecorresponding mesh openings being about 50 microns and about 40 micronsrespectively.

The wire mesh 7 may be of any suitable conductive material, such asgold, silver, copper, or nickel, the latter material being preferred. Itis suitably constructed by the process known as electroforming, forexample as manufactured by the Buckbee-Mears Company, 245 E. 6th St.,St. Paul, Minn, U.S.A. The 500 lines per-inch nickel wire mesh havingthe Ruling Number 509A in Buckbee-Mears Electroformed Mesh List datedMar. 25, 1965, has been used with special success.

The wire mesh 7 is mounted across the anode 2 in a manner that will belater described in detail and is electrically connected to said anode.According to an important feature of the invention, the dustshieldscreen '7 is positioned at a substantial axial distance from the upperend of anode 2 which constitutes the plane of the electrostatic lens,that is, at a position intermediate the plane of said lens, and theplane of the output image screen 3.

In this connection it will be understood that it would be desirable forthe purposes of the invention to position the dustshield screen 7 asclose as possible to the output screen 3 in order to protect the saidscreen against the foreign particles that are present in as large aspossible a volume of the tube envelope. However, if the dust screen 7 ispositioned too close the upper surface of the secondary screen 5, ashading effect is produced which prevents the screen areas underlyingthe mesh wires from being irradiated with photo-electrons, with adverseeffect on the image. If on the other hand the wire mesh screen ispositioned too close to the lens section defined between the ends ofelectrodes it and 2, ie. too close to the crossover point 0, the meshwill interfere with the traiectories of the electrons and will causeimage aberration. A preferred position for the wire screen i of theinvention, is such that the general plane of said screen isapproximately tangent to an equipotential surface corresponding to notless than about 90%, and preferably about 95% of the potentialdifference between the electrodes 1 and 2, as measured in the absence ofthe wire screen '7.

This last condition will be better understood from a consideration ofFIG. 2. It will be noted that the equipotential surfaces, a few of whichare shown, bulge outwardly in both directions in the manner known to betypical for electrostatic lenses of this type. The equipotentials arelabelled in percentage values referred to the total potentialdifiference (V V of the electrodes 1 and 2. The potential gradient (orelectric field intensity) is seen to drop rapidly on the output sidefrom electrode 2, as is evidenced by the increased spacing of theequipotentials. If the wire mesh screen '7 of the invention is insertedacross electrode 2 a sufficient distance beyond the lens section asshown, its presence will not appreciably distort the shape of theequipotential surfaces, because of the low field intensity in thatregion, and will, correspondingly, not appreciably disturb the paths ofthe electrons as determined by the equipotentials. As earlier said, apreferred position for dustshield screen 7 is on a plane that issubstantially tangent to the 95% equipotential surface as measured inthe absence of the screen. This last-mentioned surface is shOWn as thedotted curve E. When the screen of the invention is thus positioned, asshown in FIG. 2, its effect will be substantially merely to push thecentre part of the 95% equipotential E a small distance upward towardthe lens, to the full-line position shown. This is because the screen 7of course represents the 100% equipotential surface, since said screenis connected to the same potential as electrode 2.

It will be understood that the optimal position for the wire screendustshield member of the invention can be readily determined in thelight of the above teachings, for any individual model of tube, as by aconventional rheographic testing procedure serving to determine thepositions of the characteristic equipotential surfaces. After thepositions of the equipotentials have been determined, by a rheographicor equivalent method, the wire screen 7 is positioned so that its planeis tangential to the center of the equipotential corresponding to atleast 90%, and preferably 95%, of the voltage difference between theultimate (anode) electrode, such as 2, and the preceding electrode, suchas 1. If desired, the wire screen 7 may be positioned somewhat furtheraway from the plane of the lens, as far down as the position of the 99%equipotential, for example.

In accordance with an important feature of the invention, the wire meshdust shield disclosed above is associated with a diaphragm serving toprevent stray electrons from striking the secondary screen 3, and suchdiaphragm, in a preferred embodiment, is combined with said wire meshdust shield to serve as an annular supporting flange therefor, as willbe presently described with reference to FIGS. 3 and 4.

It should be understood that in image-converter or brightness amplifiertubes of the type to which the invention relates, a source ofinefficient operation additional to that so far considered, isconstituted by stray atomic particles, primarily electrons, generatedwithin the tube from sources other than the photocathode, whichparticles are liable to strike the secondary screen 3 and excite brightspots thereon disturbing the secondary image. Stray electrons and ionscan be produced in the tube from a number of sources. The residual gasmolecules present within the imperfectly evacuated tube envelope becomeionized and liberate electrons. Ions and electrons, including thoseproduced in the way just referred to, can excite secondary emission onstriking the electrode and other metallic surfaces in the tube,whereupon further electrons are emitted. Also, some parasiticphotoemission is inevitably present within the tube. The chief reasonfor this is that some of the photoemissive metallic vapors fromevaporator receptacles such as 14- unavoidably condense to some degreeon surfaces other than the surface of photocathode 4; also, some of thephotoemissive coating deposited on the cathode may subsequently becomedetached from it and settle elsewhere in the tube. Such parasiticphotoemissive deposits will, during the operation of the tube, becomeexcited by various sources of illumination, including the primaryradiations transmitted and/ or diffused by the photocathode, as well aslight from the glow discharges created by the electric field in thetube. Thus, stray photo-electrons are generated. A further source ofstray electrons is that produced by field emission, especially from thesharp edges of certain electrodes, as well as any asperities that mayinadvertently remain on said electrodes after manufacture. Of thevarious above sources of stray electrons, a particularly abundant sourcein many cases is constituted by the radial flange section of the firstelectrode 1, which is adjacent to and spaced from the radial flange ofthe second electrode 2 and cooperates with it to form an electrostaticlens as described above. This electrode section tends to emit a largenumber of predominantly slow electrons in all directions, both throughthe photoelectric and the secondary emissive actions described above,which are considerably promoted due to the high field intensityprevailing in that region.

It is clear that the disturbing action of the stray particles from theabove-enumerated and other sources upon the secondary image formed onscreen 3, could be very greatly diminished, if the beam of electronsstriking the screen were restricted to the useful conical beam definedby the end paths 16 referred to earlier. According to the invention,this is achieved by diaphragming the beam by means of a diaphragm 17,positioned in the cylindrical anode 2 between the crossover point 0 andthe secondary screen 3. The optimal position for such a diaphragm is thesame as the optimal position for the dust-shield screen, as earlierdetermined herein. This can be shown as follows. On the one hand, it isapparent that the screening action of the diaphragm against strayelectrons travelling in random directions at angles to the axis greaterthan the cone angle of useful beam 16, would be maximized if thediaphragm were positioned as close as possible to the crossover point 0and were formed with an aperture of diameter equal to the diameter ofthe useful beam in the plane of the diaphragm, so as to screen off allbut said useful beam. On the other hand, the closer the diaphragm ispositioned to the crossover point 0 (of lens 1-2), the greater will bethe distorting effect produced by it upon the equipotential surfaces ofthe lens, and hence also the aberration introduced into the final image.The optimal position for the diaphragm, as dictated by a compromise ofthese conflicting conditions, will therefore be on a plane tangential toon equipotential surface of the lens such as the equipotential earlierreferred to, where the voltage gradient is low enough to avoidsubstantial distortion of the equipotential surface by the diaphragm,while yet being sufliciently close to the lens section to ensure thatthe diaphragm will achieve an efiicient screening effect.

The fact that the optimal position for the diaphragm coincides with theoptimal position for the dust-shield screen, is an exceptionallyfortunate circumstance, since it enables the dust-shield and diaphragmto be constructed as a unit, with the diaphragm serving as a supportingflange for the wire mesh. As earlier indicated, the wire mesh of thedustshield screen of the invention must be extremely fine if it is to beeffective. The mesh wire will usually have a diameter of the order ofmagnitude of microns. The screen would therefore be fragile if it had alarge area, and would impart fragility to the tube. However, bycombining the wire mesh screen with the diaphragm and positioning theresulting unit at the common optimal position for both components asdefined above, it is evident that the free area of the wire screen willsimultaneously be minimized, since it will then be not substantiallylarger than the cross sectional area of the useful electron beam 16,which area is quite small at said optimal position. In this way, thecomposite screen-anddiaphragm unit will be no weaker than any of theother internal components of the brightness-amplifier tube, and a fullysatisfactory assembly is obtained.

FIG. 4 shows in somewhat greater detail a preferred construction of thedustshield and diaphragm assembly. The diaphragm here designated 52 is astainless steel plate formed with a central aperture 54 and having itsperiphery abutted against an annular shoulder 56 of the anode 2. Springmeans, not shown, may be provided for holding plate 52 in its abuttedposition. A low annular ridge 58 having a rounded profile is formed onthe upper side of plate 52 at a radial spacing from the periphery ofaperture 54, and the electroformed wire mesh 7, is extended across theridge 58, and has its outer margin pressed down against the surface ofplate 52 by means of a spot-welded presser ring 60.

In one practical embodiment, the tube has an overall length of about 290mm. and an inner envelope diameter of 195 mm. The output screen 3 has adiameter of mm. The anode 2 has its end plane (60, FIG. 4) at an axialdistance of 30 mm. from the upper surface of output screen 3, and thediameter of its aperture (64, FIG. 4) is mm. The diaphragm plate 52, of1 mm. gauge stainless steel, carrying the wiremesh dust screen 7 formedand mounted as described, was positioned with its under surface 13 mm.from the surface of output screen 3, and its center aperture 54 was 10mm. in diameter, which is just larger than the cross section diameter ofthe useful electron beam 16 at that position. The plane of the wire meshscreen 7 was in this manner positioned about 15 mm. from the upper planeof anode 2. In many cases it is convenient to define the position of thewire screen 7 in terms of the ratio A/D of the distance A from saidscreen to the top of the anode, to anode aperture diameter D. Values inthe range from 0.2 to 1.5 are generally found satisfactory for thisratio according to the invention. A preferred range is from 0.7 to 1.0.

Returning to the dust-shielding function of the wire screen of theinvention, it will be observed that owing tt. the presence of the wirescreen 7, only those floating fragments and dust particles ofappreciable size that may be present in the relatively small spacedefined between said wire screen and the secondary-image screen 3, areable to settle on the surface of said image screen and mar the imagethereon. This represents only a small fraction of the total amount ofparticles floating about in the envelope, and which would be liable tosettle on the image screen 3 in the absence of the dust-shield screen ofthe invention. Further, inasmuch as a chief source of objectionableforeign particles originating during the use of the tube are residualfragments of the contents of the evaporator receptacles 14, that failedto vaporize during the evaporation step earlier described, and whichlater become detached during handling of the tube in service, it is seenthat this source of trouble is eliminated. The same applies to fragmentsof photoemissive coating liable to be detached from the photocathode 4.An auxiliary useful function of the dustshield screen 7 is to preventfragments broken off from the phosphor coating on the upper side ofimage screen 3, if such phosphor is provided, from landing on theunderside of the photocathode 4 where they would seriously impairemission.

It should be understood that, while a typical construction of aparticular form of brightness amplifier tube has been shown anddescribed for illustrative purposes, the teachings of this invention arelargely independent from the constructional details of such tube,including the geometry, relative dimensions and shape and number ofelectrodes used in the electron-optics of the tube, and other tubecharacteristics.

What we claim is:

1. An image tube having a sealed evacuated envelope defining a path foran electron beam, a photocathode at one end of the path, an output imagescreen at the other end of the path, and electron-optical means fordirecting photo-electrons along said path from the photocathode to theimage screen, said means including an annular anode adjacent said imagescreen and a further electrode positioned ahead of the anode anddefining an electrostatic lens therewith, wherein the improvementcomprises:

a dust shield element in the form of a fine mesh, conductive wire screenextending across said anode and electrically connected thereto, and sopositioned at a substantial spacing from the general plane of said lensas to leave the equipotential surfaces of the lens generallyundisturbed.

2. An image tube according to claim 1, wherein said dust shield elementis so positioned that its general plane is substantially tangent to anequipotential surface of said lens corresponding to not less than aboutand not more than about 99% of the potential difference present acrossthe lens, as determined in the absence of said element.

3. An image tube according to claim 1, wherein said dust shield elementis so positioned that its general plane is substantially tangent to anequipotential surface of said lens corresponding to about of thepotential difference present across the lens as determined in theabsence of said element.

4. An image tube according to claim 1, wherein said element has a meshopening less than about 80 microns in width, and an optical transparencyratio not less than about 50%.

5. An image tube according to claim 4, wherein said element has a meshsize in the range of about from 250 to 750 lines per inch.

6. An image tube according to claim 1, wherein said element has a meshopening in the range of about from 40 to 50 microns, and a mesh numberof in the range of about from 400 to 500 lines per inch.

7. An image tube according to claim 1, which further includes adiaphragm made of electrically conductive material extending across saidanode adjacent to said element and electrically connected to saidelectrode and element, said diaphragm having an aperture notsubstantially larger than the cross sectional area of the usefulelectron beam.

8. An image tube having a sealed evacuated envelope defining a path foran electron beam, a photocathode at one end of the path, an output imagescreen at the other end of the path, and electron-optical means fordirecting photo-electrons along said path from the photocathode to theimage screen, said means including an annular anode adjacent said imageand a further electrode positioned ahead of the anode and defining anelectrostatic lens therewith, wherein the improvement comprises:

an annular, electricallyconductive, dustshield-and-diaphragm unitmounted across said anode and electrically connected therewith, saidunit comprising:

a transverse apertured plate mounted across the anode and having acentral aperture not substantially larger than the cross sectional areaof said useful electron beam; and

a finemesh wire screen stretched across said aperture and bonded to saidplate;

said conductive unit being so positioned at a substantial axial spacingfrom the general plane of said lens 1 1 as to leave the equipotentialsurfaces of the lens generally undisturbed.

9. An image tube according to claim 8, wherein said unit is sopositioned that its general plane is substantially tangent to anequipotential surface of said lens corresponding to not less than about90% and not more than about 99% of the potential difference presentacross the lens, as determined in the absence of said unit.

10. An image tube according to claim 8, wherein said unit is positionedat an axial spacing from said lens in the range of about from 0.2 to 1.5times the diameter of the lens aperture.

11. An image tube according to claim 8, wherein said References CitedUNITED STATES PATENTS Rotow 313-65 Niklas 313-65 X Stoudenheimer et al.313-65 Mesta 313-94 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

